The proposed clinical trial shall address whether metformin has utility in the prevention of oral cancer. It is a phase IIa clinical trial of metformin ER 2,000 mg per day given for 12-14 weeks to individuals with oral premalignant lesions, either oral leukoplakia or erythroplakia (which are precursors to oral cancer). The goal is to determine if metformin decreases the size of oral leukoplakia (clinical response). Additionally, effects on biomarkers of cancer risk and metformin effect will also be ascertained. The clinical trial shall be performed at the University of California San Diego Moores Cancer Center, the University of Minnesota, and the British Columbia Cancer Agency, with monitoring and oversight provided by the University of Arizona.